A radio frequency (RF) modulator receives a baseband signal and generates a modulated signal based thereon. For example, an RF modulator may be used to receive a baseband signal from an audio and/or video circuit. The audio and/or video circuit may be responsive to a device such as a digital video disk (DVD) player, a camcorder, a video gaming system, a videocassette recorder (VCR), a digital media player, or any other suitable device. The RF modulator may receive the baseband signal and generate a modulated signal within a particular frequency band or channel. An output device, such as an analog television, a radio, or any other suitable device, may be tuned to the particular channel in order to receive the modulated signal and generate an audio and/or video output based thereon.
The modulated signal may be divided into multiple subcarriers (e.g., a video and audio subcarrier) that operate at different frequencies within the frequency band or channel. This method, commonly referred to as Frequency Division Multiplexing (FDM), allows each subcarrier to be modulated independently and thus each subcarrier may include independent information. Common modulation techniques for each subcarrier include, but are not limited to, amplitude modulation (AM), frequency modulation (FM), quadrature amplitude modulation (QAM), quadrature phase shift keying (QPSK), and other suitable modulation techniques known in the art. Each modulated subcarrier can be combined and simultaneously transmitted within the frequency band or channel.
The subcarriers typically include harmonics that are multiples of a frequency at which they are oscillating. For example, if the subcarrier is oscillating at a fundamental frequency f, harmonics would be included at 1f, 2f, 3f, . . . , Nf. These harmonics may cause interference on adjacent subcarriers and/or frequency bands or channels, which is undesirable. It is therefore desirable to suppress harmonics that are included in the subcarriers.
One method to suppress the harmonics is to use a filter that is designed to allow the fundamental frequency to pass while blocking the others. In order to suppress low order harmonics (e.g., 2f, 3f, . . . ) of a subcarrier, a high order filter is required. However, high order filters often have variations in filtering characteristics due to manufacturing variations, which makes it difficult to suppress low order harmonics without adversely affecting the subcarrier.
Another method to suppress the harmonics is to generate a synthesized waveform based on the subcarrier that includes the audio and/or video information. FIG. 1 discloses a direct digital synthesis (DDS) circuit 10 that is operative to generate a synthesized digital waveform based on the subcarrier frequency. The circuit 10 is operative to receive a plurality of control signals 1, 2, 3, and 4 that are generated by a subcarrier oscillator (not shown). The circuit 10 is operative to generate a synthesized digital waveform of either a frequency or amplitude modulated subcarrier. For example, if a synthesized waveform of a frequency modulated subcarrier is desired, a voltage positive with respect to Vs− may be applied to terminal 12 and the subcarrier oscillator is frequency modulated. However, if a synthesized waveform of an amplitude modulated signal is desired, a voltage positive with respect to Vs− may be applied to terminal 14 and the amplitude modulated signal may be operatively coupled to the AM input.
Terminal 16 is operatively coupled to transistor 17 and is operative to receive control signal 1. Terminal 18 is operatively coupled to transistor 19 and is operative to receive control signal 2. Control signals 1 and 2 oscillate at a first frequency and are typically square waves that are 180 degrees out of phase. Terminal 20 is operatively coupled to multiple transistors generally identified at 22 and is operative to receive control signal 3. Terminal 24 is operatively coupled to multiple transistors generally identified at 26 and is operative to receive control signal 4. Control signals 3 and 4 oscillate at a second frequency and are typically square waves that are 180 degrees out of phase. During operation, the control signals 1-4 enable and disable current flow through respective transistors 17, 19, 22, 26. In addition, signals 3 and 4 enable and disable current flow I1 and I2 through transistors 27 and 29, respectively. When the transistors 17, 19, 22, 26, 27, and 29 are enabled and disabled according to control signals 1-4, currents I3 and I4 are varied generating a synthesized waveform Vout between output terminals 28 and 30. Although this circuit 10 works, it exhibits poor common mode rejection and as such variations in the power supply can appear in the output signal.
To more clearly describe the operation of the circuit 10, FIG. 2 is provided. FIG. 2 depicts exemplary waveforms of currents I3 and I4 and the synthesized waveform Vout at 50. FIG. 2 also depicts operation of the circuit 10 during time t1 to t2 at 52 and time t2 to t3 at 54. At time t1, transistor 27 is enabled allowing I1 to flow and transistor 29 is disabled. From time t1 to t2, 14 is the only current available to generate the synthesized waveform Vout. At time t2, transistor 29 is enabled allowing 12 to flow and transistor 27 is disabled. From time t2 to t3, I3 is the only current available to generate the synthesized waveform Vout. Since currents I1 and I2 are not continually flowing during operation, the circuit 10 exhibits a poor common mode rejection.
It is therefore desirable, among other things, to provide a circuit capable of generating a synthesized waveform that suppresses subcarrier harmonics and that exhibits improved common mode rejection.